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July 07, 2008

The Whole Can Be Less than the Sum – Adaptively Reducing Power

The title of this week’s blog sounds incorrect. Isn’t it called synergy when the action of combining pieces together results in something greater than all the parts? This is usually true, but today I’m going to discuss increasing energy efficiency through active means - that is, we’ll discuss what happens when you combine parts together in a design and the resulting power consumption of the system is actually lower (a great deal lower) than that of all the individual components. How can this be? It’s actually quite simple, so let’s take a look.

Here’s the concept - put together a bunch of components that monitor some power-consuming process and continuously "adapt" to the current required conditions to lower the energy consumed. An example could be the back-light power supply for a personal media player. While watching a video the player’s power supply is fed ambient light information from a photodiode. As the ambient light changes, the drive current to the white LED backlight is adjusted. It rarely needs to be at full power (direct sun light), so it adapts to the current surroundings. In this way, the battery life is increased and the total energy consumed is reduced.

Now you may argue, "yes, but in direct sunlight the overall power consumption is actually larger due to the additional circuitry required to monitor the ambient light". I will concede that argument is true. However, let’s use some statistics (I love math!) to prove my point. My theory is that if you were to create a usage pattern of the ambient lighting conditions of a large number of personal mobile devices (especially media players), you’d find most of them (around 84%) are watching their video in much less than full sunlight. I arrived at this fairly large number by assuming a Gaussian distribution of the ambient lighting conditions that most people use - completely dark to full sunlight. If I add 1 standard deviation on either side of the mean and then add in the remaining distribution on "the dark side" (couldn’t resist the pun), you calculate about 84% of the time users are not watching the video in full sun (or near full sun). Actually most of the time users are in less than full sun for other reasons such as preventing a sun burn or simply being comfortable - I live in Florida and I should know!

So if you accept my theory on the usage patterns, then you must agree that if a PMP is simply designed for full sunlight viewing, it will use considerably more power then the device designed to "adapt" to the ambient conditions. The next question is how much power is saved by being adaptive... this requires a bit more math. First, let’s assume again a Gaussian distribution of light conditions during playback over normal usage. We’ll use the standard Gaussian distribution formula as part of our calculations shown in Equation 1.

We’ll also assume a 3σ (standard deviations) spread to set the limits for full darkness and full brightness (daylight viewing with the LEDs at 100%). This will include 99.7% of the usage cases (with a 0.3% error). To clarify, -3σ = 0% brightness and +3σ = 100% brightness (see drawing 1 and equation 2). We will also assume a continuously variable drive to the LED back-lights (as apposed to a stepped approach) based on the ambient conditions that will never drop below 20% even in complete darkness. By applying the backlight level function in equation 2 to our distribution function shown in equation 1, we can then calculate the total percentage power used by the adaptive backlight. Equation 3 shows the total power calculation. This calculation is for the entire probability distribution including very bright and full sun conditions. If we evaluate the integral from -3σ to 1σ which represents 84% of the population (as mentioned before), the power is reduced from 59.8% down to 47.1% - an incredible savings.

Now let’s take a look at the system impact of the back-light savings on the total run time of the device. Assume the back-light LEDs represent 40% of the total power consumption of the device when at full brightness. Assume a run time of 2 hours based on a non-adaptive back-light. If we reduce the backlight power by 40% for all users, then the overall system run time improves to 2 hours and 23 minutes. That’s an overall system improvement of 19%. Now, if we reduce the backlight consumption by 53% for 84% of the population that never watch video in bright light, the run time goes up to 2 hours and 32 minutes - a 27% improvement in performance.

In this discussion we have not considered the power consumed by the adaptive circuit, so we’ll assume it’s negligible relative to the backlight. In many cases it takes very little additional circuitry to perform these types of tasks. As you can imagine, if you are watching video in almost complete darkness with this adaptive PMP, then you could probably get an additional hour of play from it. Just some thought provoking ideas... If you want to know more about adaptive power reduction, check out National’s PowerWise® Solutions page at http://www.national.com/powerwise.